Original title: [Navigation Forum] BeiDou 3 satellite navigation signal and reception strategy
editor's commentary
by"5G/At the summit forum themed "GNSS High-Precision Positioning and Deep Application of Spatial Data", Prof. Lu Mingquan from Tsinghua University brought a keynote report titled "BeiDou 3 Satellite Navigation Signal and Reception Strategy" to the delegates.
Professor Lu firstly made a brief note on the title of the report, emphasizing the important position and significance of satellite navigation signals in system construction, application promotion and industrialization. Speaking of satellite navigation, we often talk about satellite navigation systems, such as GPS, GLONASS, Galileo, and of course, our BeiDou; we also talk about the terminal products of satellite navigation, such as chips, modules, boards, and complete machines; and we also talk about various applications of satellite navigation, such as high speed, high speed, and high speed.accurateof positioning, intelligent navigation, accurate timing and so on. However, except for experts in the industry, few people talk about the signals of satellite navigation systems. In fact, although the satellite navigation signal is invisible and untouchable, it is very important. To summarize, we can say this: satellite navigation signal is the core technology of satellite navigation system, which is listed as one of the three core technologies of Beidou 3 in China, and is not only an important part of the satellite navigation system, but also the key to the promotion and industrialization of satellite navigation applications. In fact, whether it is the construction of the system, or the development of terminal products, or a variety of applications, are essentially centered around the satellite navigation signal. The rationale is simple, because the satellite navigation system is an information system, the signal is the carrier of information, and its core is of course the signal.
Next, Prof. Lu's lecture centered on three elements:
I. Characteristics of the Beidou-3 satellite navigation signal
From the early 1990s, when the Beidou-1 project was launched, China's Beidou satellite navigation system has gone through more than 20 years of development. According to the three-step development plan, it has gone through the two stages of BeiDou-1 and BeiDou-2, and is now developing towards BeiDou-3. From the user's point of view, we can find that the development history from Beidou 1, Beidou 2 to Beidou 3 is actually the evolution history of Beidou satellite navigation signals.
BeiDou-1 is China's first satellite navigation system, covering China and its neighboring areas, and adopting the RDSS system to provide active positioning, short message communication and one- and two-way timing services. The navigation signal of BeiDou-1 consists of transmitting and receiving two kinds of signals, i.e. the inbound signal in L-band and the outbound signal in S-band. These two signals adopt very simple and mature BPSK and QPSK modulation respectively.
BeiDou-2 is a regional satellite navigation system that has been built and is being used on a large scale in China, inheriting the RDSS of BeiDou-1 and adopting the RNSS system, the so-called passive positioning technology, which is the mainstream of international GNSS. BeiDou-2 has 14 satellites in orbit, consisting of two kinds of satellites, GEO and MEO, but it is also a regional system covering the Asia-Pacific region, broadcasting military and civil signals at three frequency points, namely B1, B2 and B3. Among them, the civil signals that have been announced are B1I and B2I, and actually there is another civil signal B3I, which has not been formally announced yet, but many manufacturers and industrial users are already using it, and all kinds of high-precision receivers are basically using BeiDou signals including B3I. BeiDou-2 is the first system in the world to provide three-frequency public service, which is a major feature of the BeiDou system. It can be seen that although the navigation signals of BeiDou-2 system are numerous, it still adopts very simple and mature BPSK and QPSK type of modulation in technology. It is basically the same as the early GPS signals.
According to the three-step development strategy, what is currently being deployed is Beidou III. At the beginning of last month, that is, on September 5, the official website of BeiDou released a news with less than 200 words, and attached to it was an 80-page document, which is the "BeiDou Satellite Navigation System Space Signal Interface Control Document for Open Service Signal B1C, B2a (Beta Version)". Although the release of this ICD is unusually low-profile, it is an important milestone in the history of the development of the BeiDou satellite navigation system, which not only unveils the official deployment of the BeiDou-3 system, but also finally unveils the BeiDou-3 navigation signals. According to this ICD beta version, we can learn some basic information about BeiDou 3, including constellation structure, navigation signals, service functions, etc., but the service performance has not yet been announced. The important information that has been made clear are:
(1) The Beidou-3 constellation has a total of 30 working satellites, including 3 GEOs, 3 IGSOs and 24 MEOs, and will deploy in-orbit backup satellites as appropriate;
(2) B1C and B2a signals are broadcasted on BeiDou 3 MEO and IGSO satellites to provide public services;
(3) The B1C signal is a new signal, and the B2a signal will replace the B2I signal.The B1I signal is broadcast on all BeiDou 3 satellites to provide public service;
(4) GEO satellites will provide SBAS services.
It can be seen that BeiDou 3 will provide public services by at least three signals, namely, B1C, B1I and B2a. The new navigation signals B1C and B2a will be compatible and interoperable with GPS and Galileo, which means that BeiDou 3 will be further integrated into the family of international GNSS, and will also bring about a major change in satellite navigation receiver technology, and the performance of future services will be greatly improved, and the power consumption and cost of user equipment will be significantly reduced. The power consumption and cost of user equipment will be significantly reduced. In particular, it should be pointed out that continuing to broadcast B1I signals will not only ensure a smooth transition from BeiDou-2 to BeiDou-3, but will also protect the interests of receiver manufacturers and users to the greatest extent possible.
The following highlights the technical characteristics of the new Beidou 3 signal.
B1C is the main signal of BeiDou 3, which is required to be received by all BeiDou users and even global GNSS users in the future, and will become an important symbol of the BeiDou system (similar to the L1 C/A of the current GPS and the L1C signal in the future).B1C is a new-generation navigation signal with advanced technology and independent intellectual property rights, and it can not only satisfy the needs of low-cost users of the consumer category such as location services, but also meet the needs of high-performance users of the professional category such as high-precision measurement. B1C is a new generation navigation signal with advanced technology and independent intellectual property rights, which can meet the needs of both consumer low-cost users, such as location services, and professional high-performance users, such as high-precision measurement.
The carrier frequency of the B1C signal is 1575.42 MHz, which is shared with GPS L1 and GalileoE1, and the bandwidth is 32.736 MHz.A modernized signal structure with quadrature between data and pilot frequency is adopted: the data component is generated by the navigation message and ranging code via subcarrier modulation, and is modulated by sinusoidal BOC(1, 1) modulation; the pilot frequency component is generated by the ranging code via subcarrier modulation, and is modulated by QMBOC(6, 1,4/33) modulation. The power ratio of data component and guide frequency component is 1:3, and the signal power is tilted to the guide frequency, which is in line with the design principle of the higher ranging accuracy, the better, and the demodulation can be used, which is conducive to improving the overall performance of the B1C signal.
The ranging code structure of the B1C signal is the same as that of B2a, and both consist of a main code and a subcode heterodyne. The main code has a rate of 1.023Mcps and a code length of 10230, which is obtained by truncating the Weil code of length 10243. There are 126 main codes, i.e., 63 data codes and 63 conduction frequency codes.The sub-code of B1C conduction frequency component is 1800 in length, which is obtained by truncation of Weil code of 3607 in length, and is generated in the same way as the main code, with a total of 63 codes.
The navigation message for the B1C signal is in B-CNAV1 format.The B-CNAV1 navigation message data is modulated on the B1C data component with a message length of 1800 symbol bits per frame, a symbol rate of 100 sps, and a broadcast period of 18 seconds.
B2a is the second civil signal of BeiDou-3, which is used to replace the B2I signal of BeiDou-2, and mainly provides services for dual-frequency or triple-frequency receivers, which can be used for high-performance services such as life safety services and high-precision measurements, as well as for consumer services with higher performance requirements.
The B2a signal carrier frequency is 1176.45 MHz, shared with GPSL5 and Galileo E5a, with a bandwidth of 20.46 MHz, and also adopts the structure of data-conducted-frequency quadrature (QPSK): the data component is generated by the modulation of the navigation message data and the ranging code, and modulated by BPSK(10); the conducted-frequency component consists only of the ranging code, and is also modulated by BPSK(10). BPSK(10) modulation is also used. The power ratio of the frequency-guided component to the data component is 1:1.
As already mentioned, the ranging code structure of the B2a signal is the same as that of the B1C, which also consists of the main code and the subcode heterodyne. The main code rate is 10.23Mcps and the code length is 10230, which is obtained from two 13-stage linear feedback shift registers by shifting and modulo two-sum generation of Gold code expansion. On the same satellite, the main code generation polynomials for the two components of the B2a signal are different, but the same initial state is used.There are a total of 126 ranging codes for the B2a signal, of which there are 63 each for the data code and the frequency guide code. For different satellites, the subcodes of the B2a data component are the same, and the subcodes of the B2a frequency guide component are different.The code length of the B2a data component is 5, and a fixed 5-bit code sequence is used as the subcodes, and the subcode sequence is 00010.The code length of the B2a frequency guide component is 100, which is obtained by truncation of the Weil code of the length of 1021, and the definition is same as that of the B1C master code.
The navigation message for B2a signals is in B-CNAV2 format.The B-CNAV2 navigation message data is modulated on the B2a data component, with a message length of 600 symbol bits per frame, a symbol rate of 200sps, and a broadcast period of 3 s. Up to 63 message types can be defined in the B-CNAV2 navigation message, and 7 valid message types are currently defined.
II. Strategies for the reception of satellite navigation signals
B1C is a brand-new navigation signal with advanced technology, complex structure and more signal components, and a variety of different reception schemes can be developed to meet different user needs. Moreover, BeiDou 3 will broadcast B1C and B1I on MEO and IGSO satellites at the same time, which will develop unique reception methods and can fully exploit the potential of BeiDou 3. B2a signal is a composite signal orthogonal to the data and the guide frequency, which is similar to the GPS L5 and Galileo E5a, and the basic reception method is already mature. Therefore, the main innovation of future BeiDou-3 reception technology lies in the new theory and method of B1C signal reception processing, as well as the efficient interoperability reception technology of B1C with GPS L1C and Galileo E1 OS signals. This is what will be introduced next in this report.
Let's first analyze the structural characteristics of the B1C signal. It has already been talked about that the B1C signal consists of the BOC(1, 1) data component and the BOC(6, 1, 4/33) guide frequency component, where the BOC(6, 1, 4/33) component contains the orthogonal BOC(1, 1) and BOC(6, 1) components. Thus, the entire B1C signal actually contains three real components: the BOC(1, 1) data, the BOC(1, 1) derivative, and the BOC(6, 1) derivative.
From this, from a methodological point of view, we can develop two basic reception methods: wideband reception and narrowband reception.
Wideband reception: the bandwidth is taken to be around 14MHz, and the narrowband component BOC(1, 1) and the wideband component BOC(6, 1) are received simultaneously. In this matched reception mode, since the QMBOC of the B1C has the same power spectral density as the TMBOC and the CMBOC, the three have exactly the same capture and tracking performance.
Narrowband reception: the bandwidth is taken around 4MHz and only the narrowband component BOC(1, 1) is received. When the receiver only processes the BOC(1,1) component, QMBOC has better performance: the capture sensitivity of QMBOC is more than 0.51 dB higher than that of TMBOC; the QMBOC performance is 0.6164 dB higher than that of TMBOC when the receiving bandwidth is 4 MHz.
Therefore, overall QMBOC outperforms GPS and Galileo's TMBOC and CMBOC signals.
Since BeiDou 3 will broadcast B1C and B1I on MEO and IGSO satellites at the same time, and B1I and B1C are generated separately based on the same star clock, and then transmitted to the ground via amplifier and antenna after a special multiplexing scheme, B1C and B1I can be regarded as a special bilateral broadband signal from the user's point of view. We tentatively call it asymmetric bilateral band signal B1-ADS at B1 frequency point.The characteristics of B1-ADS signal are:
(1) Very high equivalent RMS bandwidth: B1-ADS equivalent bandwidth is not only larger than that of B1I and B1C, but even larger than that of B2a signal using 10.23 Mcps broadband. Therefore, the B1-ADS signal has significant theoretical advantages in ranging performance, as well as obvious anti-interference and anti-polygon advantages.
(2) Complex autocorrelation function: the autocorrelation function of B1-ADS has a very sharp main peak, which indicates that the B1-ADS signal theoretically has a significant ranging performance advantage, but it also has a more complex side-peak structure, which needs to be solved in the reception of the complex multi-peak blurring problem.
Since the modulation, code rate, message and power of the two sidebands B1I and B1C signals of B1-ADS are different, the traditional processing techniques used for symmetric signal reception are not applicable. Therefore, we need to develop a special class of reception method to receive B1-ADS, i.e., to receive B1I and B1C at the same time, in order to further exploit the potential of ranging accuracy and anti-jamming and anti-polygraphy of BeiDou-3 signals. The following figure shows a proposed B1-ADS reception scheme, which is the joint reception scheme of B1I and B1C signals.
The basic ideas of this scheme are: drawing on DET to independently track the subcarrier to solve the multi-peak blurring; correlating the upper and lower sidebands separately to avoid generating the subcarrier waveforms and flexibly supporting the double and single sideband processing; adopting correlators with different structures in the upper and lower sidebands to solve the problem of different modulation modes in the upper and lower sidebands; utilizing the known transmit power ratio to perform power compensation and solving the problem of different power in the upper and lower sidebands; and utilizing the data-assisted message stripping method to solve the B1I data-only channel problem.
This approach can support quasi-optimal receive processing of B1-ADS signals with low hardware complexity, in addition to being compatible with B1I or B1C independent receive processing modes.
III. Future trends of Beidou and GNSS receivers
Around 2020, the modernization of GPS will enter the GPS III phase, the deployment of Galileo will be basically completed, and our BeiDou III will also be deployed. This means that even without counting the satellites of GLONASS, there are more than 90 satellites in orbit for BeiDou III and the three major systems of GPS and Galileo, and in addition to the six interoperable signals at the two frequency points of L1/E1/B1 and L5/E5a/B2a, which will become the mainstream, there will be other signals available to users. The global GNSS resources will be unprecedentedly abundant, especially in our country.
At that time, although the satellite navigation resources are very rich, that is, the user can receive a very large number of signals, but if you accept all the signals as they are, then the cost of the receiver is too great. Today's invited report has been discussing the need to achieve high accuracy at low cost, and it is obvious that the simple way of receiving all signals is difficult to achieve high accuracy at low cost. Fortunately, the mainstream signal of future GNSS is the new generation of interoperable signals, and the broadcasting of interoperable signals brings us a reasonable way to realize high accuracy positioning at low cost: selectively receiving interoperable signals from multiple systems, and developing a new generation of interoperable receivers. This will also be the main development direction of future satellite navigation receivers.
The basic concept of interoperable receivers is to make full use of the similarity of the interoperable signals of different navigation systems, to reduce the complexity of receivers by sharing antennas and RFs, multiplexing capture and tracking channels, and to achieve the purpose of performance enhancement, power consumption reduction, and cost reduction. It can be expected that the future interoperable receiver will have three basic forms: one is a single-frequency multi-system interoperable receiver, the second is a dual-frequency multi-system interoperable receiver, and the third is a multi-frequency multi-system interoperable receiver.
Single-frequency multi-system interoperable receiver: it only receives the interoperable signals of L1/E1/B1 frequency points, mainly including L1C, E1 OS and B1C, and it will likely become a low-cost, low-power receiver with the most popular application in the future. Many current consumer single-frequency multi-system receivers are actually triple-frequency receivers, i.e., they receive GPS L1 C/A, BeiDou B1I and GLONASSG1 at the same time, and the hardware is still relatively complex. By around 2020, the new generation of single-frequency multi-system interoperable receivers can realize true single-frequency reception, and the performance will definitely be better, and the power consumption and cost are greatly reduced.
Dual-frequency multi-system interoperable receiver: The current dual-frequency multi-system receiver does not belong to the category of interoperable receivers. The future will develop a receive B1C and B2a, L1C and L5, E1 OS and E5a interoperable dual-frequency receiver, both can become a highly reliable, high-performance dual-frequency receiver mainstream products, but also with the development of technology will soon enter the field of high-performance consumer. This type of receiver is a new generation of interoperable signals put into use the inevitable product, is a new class of receivers from scratch, after the star-based enhancement, real-time positioning accuracy can reach the decimeter level, will play an important role in the application of high security and reliability requirements of intelligent driving, machine control, infrastructure timing and high-performance navigation and positioning. In addition, it can also be applied in the field of high-precision measurement and consumer areas with high performance requirements. Dual-frequency interoperable receivers are the focus of future receiver development, and the latest release of products also supports this view. Last month, a foreign company released a new dual-frequency SoC chip, which is said to be able to achieve 30 cm accuracy in the case of dual-frequency reception, and very low power consumption. Domestic manufacturers have also launched a similar product to support the new signal of Beidou 3.
The third category is to high-precision measurement-based high-precision multi-frequency interoperable receivers: these receivers to receive dual-frequency interoperable signals, plus a third frequency signal. The emergence of this type of receiver, will make the current high-precision measurement type receiver manufacturers one of the watchword - semi-codeless and codeless receiving technology gradually fade out of the market, will trigger another round of high-precision receiver market technology competition. Due to the reception of interoperable signals, to avoid the drawbacks of code-free and semi-code-free technology, a new generation of multi-frequency interoperable receivers to reduce costs and power consumption while significantly improving the high-precision positioning performance, will become the mainstream of the future of high-precision measurement receivers.
IV. Conclusion
As the only interface between the satellite navigation system and the receiver, the satellite navigation signal is not only an important part of the satellite system, but also the link to build the entire satellite navigation industry chain from chips, boards, machines to various application solutions, and the carrier of various positioning, navigation and timing applications. Therefore, the satellite navigation signal has actually become the most important symbol of the upgrading of the satellite navigation system, and has even become an important symbol of the satellite navigation system.
BeiDou 3 will soon broadcast a brand new satellite navigation signal with advanced technology, excellent performance, excellent compatibility and interoperability with GPS and Galileo, and possesses independent intellectual property rights, representing the leading level of international GNSS, which not only lays a solid foundation for the deployment of BeiDou 3, the promotion of its applications and the sustainable development of China's satellite navigation industry, but also provides an important guarantee for BeiDou 3 to go global and become an international mainstream. It also provides an important guarantee for Beidou 3 to go to the world and become the international mainstream.
With the deployment of the BeiDou-3 system and the launching of the new signal of BeiDou-3, China's satellite navigation technology, application and industry have finally ushered in an excellent opportunity to stand on the same starting line with the developed countries in Europe and America, and it will surely become an important milestone for China to catch up with the world's advanced level in the field of satellite navigation.
Author's Profile
Mingquan Lu, Ph.D., is a professor and doctoral supervisor of the Department of Electronic Engineering at Tsinghua University, as well as the director of the Institute of Information Systems, the director of the Center for Positioning Navigation and Timing (PNT), and a special expert of the Think Tank of the Shanghai Beidou Navigation Innovation Institute. For many years, he has been engaged in teaching and scientific research in Global Navigation Satellite System (GNSS) and related fields. In recent years, he has devoted himself to the research of GNSS signal design and reception processing, GNSS system simulation and performance evaluation, as well as new technologies of positioning, navigation and timing, etc. His major achievements include the design of new-generation BDS navigation signals, the development of advanced receivers for BDS/GNSS, and the design of new regimes of the ground-based regional positioning system (LPS), etc. He has been invited to participate in conferences and seminars at home and abroad. He has published more than 150 papers in academic journals and conferences at home and abroad, 3 books, 1 conference proceedings, and more than 20 domestic and foreign invention patents. He is also a member of the expert group of the Second Generation Navigation Special Project, a member of the BeiDou Standardization Technical Committee, an editorial board member of several journals, and an academic member of several key laboratories.
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